The present invention relates to a method for detecting deterioration of a catalyst and measuring the conversion efficiency thereof with an air/fuel ratio sensor.
A conventional method detects deterioration of a catalyst based on the difference between the maximum output voltages of two oxygen sensors disposed on upstream and downstream sides of the catalyst (Japanese Published Unexamined Patent Application No. Sho-63-231252).
The output characteristic of an oxygen sensor changes abruptly around .lambda.=1. In other words, as shown in FIG. 7, the waveform of the output switches between a lean air/fuel ratio and a rich air/fuel ratio with a steep slope and does not show an intermediate or mean value. Accordingly, in the conventional method for detecting deterioration of a catalyst with two oxygen sensors, processing of comparison data between the two waveforms is rather troublesome and complicated. Moreover, accurate and precise detection is difficult for a converted rate of lower than 70% as seen in FIG. 8.